1. Field of the Invention
The invention relates to a micro-machine switch and a method of fabricating the same, and more particularly to a micro-machine switch which can conduct on-off control in a current ranging from a direct current (DC) to an alternating current having a frequency of GHz or greater, and a method of fabricating the same.
2. Description of the Related Art
There have been suggested many micro-machines. Hereinbelow is explained a micro-machines switch, as an example, suggested in Japanese Unexamined Patent Publication No. 9-17300.
FIG. 1A is a plan view of the micro-machine switch suggested in the Publication, and FIG. 1B is a longitudinal cross-sectional view taken along the line E-E′ in FIG. 1A.
As illustrated in FIGS. 1A and 1B, an anchor structure 52 composed of thermosetting polyimide, a lower electrode 53 composed of gold, and a pair of signal lines 54 composed of gold are formed on a substrate 51 composed of gallium arsenide. As illustrated in FIG. 1A, the pair of signal lines 54 is arranged such that ends of the signal lines spaced away from and faces each other.
A cantilever 55 composed of a silicon dioxide film is supported on the anchor structure 52. The cantilever 55 extends to the signal lines 54 beyond the lower electrode 53, and faces both the lower electrode 53 and the signal lines 54 with a spatial gap therebetween.
An upper electrode 56 composed of aluminum is formed on an upper surface of the cantilever 55. The upper electrode 56 extends from the anchor structure 52 to a location at which the upper electrode 56 faces the lower electrode 53.
A contact electrode 57 is formed on a lower surface of the cantilever 55 in facing relation with the signal lines 64.
The micro-machine switch having the above-mentioned structure operates as follows.
When a voltage of 30V is applied across the upper electrode 56 and the lower electrode 53, an electrostatic force is exerted on the upper electrode 56 as an attractive force towards the substrate 51 (downwardly along an arrow 58). As a result, the cantilever 55 is deformed towards the substrate 51 with the anchor structure 52 acting as a fulcrum, thereby the contact electrode 57 makes contact with the ends of the signal lines 54.
In a normal condition, as illustrated in FIG. 1B, there is a gap between the contact electrode 57 and the signal lines 54, and the signal lines 54 are spaced away from each other. Hence, in a condition in which a voltage is not applied to the lower electrode 53, a current does not run through the signal lines 54.
In contrast, when a voltage is applied to the lower electrode 53, and resultingly, the contact electrode 57 makes contact with the signal lines 54, the signal lines 54 make electrical contact with each other through the contact electrode 57. Thus, a current runs through the signal lines 54.
As mentioned above, by applying a voltage to the lower electrode 53 or stopping application of a voltage to the lower electrode 53, it is possible to cause a current to run or not to run through the signal lines 54, or control on/off of a signal running through the signal lines 54.
In order to reduce a loss in the switch, it is important to sufficiently electrically insulate the upper electrode 56 and the contact electrode 57 from each other. If the upper electrode 56 and the contact electrode 57 are electrically connected to each other, a signal (including a direct current) running through the signal lines 54 would run also through the upper electrode 56, resulting in deterioration in switch characteristics.
Even if the upper electrode and the contact electrode 57 are not electrically connected to each other, an alternating current running through the signal lines 54 runs further through the upper electrode 56 under a circumstance where electrostatic capacity is so great.
As mentioned above, if the upper electrode 56 and the contact electrode 57 are not sufficiently electrically insulated from each other, a signal leaks out of the signal lines 54, resulting in deterioration in switch characteristics.
It was found out that the conventional micro-machine switch illustrated in FIGS. 1A and 1B is accompanied with the following problems.
It is necessary to apply a sufficient voltage across the upper electrode 56 and the lower electrode 53 such that a resultant electrostatic force overcomes a restoring force of the cantilever 55, in order to turn the switch on, namely, to cause the contact electrode 57 to make contact with the signal lines 54. A produced electrostatic force is in inverse proportion to (D−W)2 wherein D indicates a distance between the upper electrode 56 and the lower electrode 53, and W indicates a thickness of the cantilever 55. Accordingly, since the thickness W of the cantilever 55 is constant, it is important to design the distance D as small as possible.
In the micro-machine switch illustrated in FIGS. 1A and 1B, the distance D between the upper electrode 56 and the lower electrode 53 is greater than a sum of the thickness W of the cantilever 55 and a distance S between the contact electrode 57 and the signal lines 54 by a thickness Wa of the contact electrode 57, as illustrated in FIG. 1B.
For instance, in order to reduce a loss in a signal in application of a micro-machine switch to a radio-frequency, it would be necessary to design the contact electrode 57 and the signal lines 54 to have a thickness of about 2 μm.
It would be necessary to space the signal lines 54 and the contact electrode 57 from each other by 4 μm or greater, in order to reduce electrostatic capacity coupling between the signal lines 54 and the contact electrode 57 when the switch is off. Hence, the upper electrode 56 and the lower electrode 53 are spaced away from each other by a sum of the thickness W of the cantilever 55 and 6 μm (4 μm+2 μm).
It was found out by the inventors that a high voltage, specifically, a voltage of about 100V has to be applied across the upper electrode 56 and the lower electrode 53, when an area in which the upper electrode 56 overlaps the lower electrode 53 is equal to 10,000 μm2, and the cantilever 55 has a width of 20 μm and a length of 130 μm.
Though it is possible to reduce a restoring force of the cantilever 55 by designing the cantilever 55 to have a greater length or a smaller width, such increasing a length or decreasing a width might cause breakage of the cantilever 55 during fabrication of a device or during operation of a device.
On the other hand, it would be possible to generate a greater electrostatic force, and hence, reduce an applied voltage by increasing an area in which the upper electrode 56 and the lower electrode 53 overlap each other. However, this causes an increase in a size of a device.
In the conventional micro-machine switches, an applied voltage is reduced in accordance with the above-mentioned method. However, the above-mentioned method is accompanied with a problem that a device would have an increased size. Thus, there is a limit in the conventional methods to reduce a size of the micro-machine switch.
In addition, since a high voltage, specifically, a voltage of about 100V is applied across the upper and lower electrodes, the cantilever 55 would be required to be composed of a qualified film, in order to prevent a device from being destroyed by dielectric breakdown.
However, in the method in which the cantilever 65 in the conventional micro-machine switch illustrated in FIGS. 1A and 1B is formed of an oxide film on the contact electrode 57 composed of gold, by low-temperature deposition process (plasma-enhanced CVD process to be carried out at 250 degrees centigrade or smaller), it would be quite difficult or almost impossible to form such an oxide film having a sufficient resistance to a voltage.
In addition, as an applied voltage becomes high, power consumption in a driver circuit increases. It was found out that the micro-machine switch could not be applied to an antenna having a plurality of switches, in particular.
Many micro-machine switches have been suggested in addition to the above-mentioned micro-machine switch illustrated in FIGS. 1A and 1B.
For instance, Japanese Unexamined Patent Publication No. 2-260333 has suggested a method of fabricating a micro-machine switch, comprising the steps of forming an epitaxial layer having a second electrical conductivity, on a silicon semiconductor substrate having a first electrical conductivity, forming a silicon dioxide film on the epitaxial layer, removing the silicon dioxide film in a predetermined area to thereby form an opening, and partially removing the epitaxial layer located below the silicon dioxide film, through the opening, by electrochemical etching.
Japanese Unexamined Patent Publication No. 6-267926 has suggested a method of fabricating a micro-machine switch, including the step of etching through the use of a silicon nitride layer as an etching mask or an etching stopper layer. The silicon nitride layer has a stress a defined by the following equation.|σ|≦3×109 dyn/cm2 
Japanese Patent No. 2693065 (Japanese Unexamined Patent Publication No. 4-306520) has suggested a micro-machine switch comprised of a substrate composed of dielectric material and having a planar surface, first and second transmission line segments formed on the substrate and spaced away from each other, a hub attached to a surface of the substrate, a switch blade fastened to the hub such that the switch blade is rotatable around the hub, and a control pad formed on the substrate, and receiving control signals to generate an electrostatic field for rotating the switch blade between an open position and a closed position. The switch blade is composed of an electrically conductive material, and has such a dimension that the switch blade electrically closes a circuit electrically connecting the first and second transmission line segments to each other, when the switch blade is rotated to the closed position.
Japanese Unexamined Patent Publication No. 4-370622 has suggested an electrostatic relay including a movable substrate and a fixed substrate both of which are composed of an electrically conductive material.
Japanese Unexamined Patent Publication No. 9-213191 has suggested a micro-machine switch comprised of a support beam comprised of a connector fixed on an insulating substrate and a movable supporter spaced away from the insulating substrate by a certain gap, a first electrode formed on the insulating substrate below the movable supporter, and a second electrode formed on a surface of the movable supporter in facing relation with the first electrode, and spaced away from the first electrode by a certain gap.
Japanese Unexamined Patent Publication No. 8-509093 has suggested a relay comprised of a first substrate formed with a cavity and composed of polycrystal material, a bridge formed across the cavity on the first substrate, a first electrical contact formed at a center of the bridge, a second substrate including a second electrical contact arranged to be engaged to the first electrical contact, and composed of insulating material, means for displacing the second electrical contact from the first electrical contact, and means for selectively moving the bridge to thereby engage the first and second electrical contacts to each other.
Japanese Unexamined Patent Publication No. 10-149757 has suggested a semiconductor micro-relay including a driver, a resistance type potential dividing circuit, and a field effect transistor FET. A movable portion is carried at the driver by means of at least one hinge such that the movable portion is elastically deformable in a thickness-wise direction of the semiconductor micro-relay. A fixed driver electrode is formed on one of surfaces of the driver which surfaces face upper and lower surfaces of the movable portion. A strain resistor is formed on at least one hinge of the movable portion. The movable portion and the fixed driver electrode are electrically connected to input terminals of the driver. The resistance type potential dividing circuit is electrically connected to the strain resistor. The resistance type potential dividing circuit is electrically connected to both a gate and an output terminal of FET.
Japanese Unexamined Patent Publication No. 8-213803 has suggested a phase-shifter comprised of at least one switch including a plurality of switchable phase-shifting devices arranged in series, adjacent phase-shifting devices being electrically connected to each other, each of the phase-shifting devices including a deflectable device. The switch determines a degree of phase shifting to a signal running through the phase-shifting devices.
Japanese Unexamined Patent Publication No. 11-204013 has suggested an electrostatic movable contact device comprised of a movable driver electrode formed on a substrate, a fixed contact electrode formed on the substrate as a fixed contact, a supporting beam partially fixed to the substrate, a movable attractive electrode formed on the supporting beam in facing relation with the movable driver electrode, and a movable contact electrode formed on the supporting beam as a movable contact in facing relation with the fixed contact electrode. The supporting beam is moved by an electrostatic force generated between the movable driver electrode and the movable attractive electrode to thereby open or close a contact comprised of the fixed contact electrode and the movable contact electrode. A plurality of the movable driver electrodes is formed on the substrate, and has the same area.
Japanese Unexamined Patent Publication No. 5-2972 has suggested an electrostatic relay including a movable porting having both a movable contact formed on a lower surface of a first substrate and a fixed contact formed on an upper surface of a second substrate, and a supporting portion which supports the movable portion so that the movable portion is movable for connecting the movable contact to the fixed contact and disconnecting the movable contact from the fixed contact. The first and second substrates are positioned such that the movable and fixed contacts face each other. The movable portion is moved by an electrostatic force generated by application of a drive voltage across the driver electrodes of the first and second substrates. The first and second are composed of an electrically conductive material, and a relay driver circuit is formed on the first or second substrate.
Japanese Unexamined Patent Publication No. 5-2973 has suggested an electrostatic relay including a movable porting having both a movable contact formed on a lower surface of a first substrate and a fixed contact formed on an upper surface of a second substrate, and a supporting portion which supports the movable portion so that the movable portion is movable for connecting the movable contact to the fixed contact and disconnecting the movable contact from the fixed contact. The first and second substrates are positioned such that the movable and fixed contacts face each other. The movable portion is moved by an electrostatic force generated by application of a drive voltage across the driver electrodes of the first and second substrates. The first and second are composed, of an electrically conductive material, and an electret is formed between the first and second substrates for strengthening an electrostatic force by which the movable portion is moved.
Japanese Unexamined Patent Publication No. 5-54782 has suggested a micro-machine comprised of a fixed electrode layer formed on a principal plane of an electrically insulating substrate, and a movable piece composed of semiconductor single crystal and positioned in facing relation with the fixed electrode layer. The fixed electrode layer and the movable piece define opposing electrodes to be driven. The micro-machine is driven by an electrostatic attractive force generated by applying a dc voltage across the opposing electrodes. At least one of the fixed electrode layer and the movable piece is roughened at surfaces thereof facing each other.
Japanese Unexamined Patent Publication No. 7-14490 has suggested an electrostatically driven relay comprised of a fixed piece defining a fixed electrode, and a movable piece including a movable electrode facing and spaced away from the fixed electrode. The movable pieced is fixed at one end such that the other end moves towards the fixed electrode by an electrostatic force generated when an external voltage is applied across the fixed electrode and the movable electrode. Contacts which makes contacts with each other or separates from each other in accordance with movement of the movable piece are formed at both the end of the movable piece and an end of the fixed piece, associated with the end of the movable piece. The contacts are electrically connected to an external electric circuit. The movable piece is outwardly rounded at proximate ends of the other end.
Japanese Unexamined Patent Publication No. 9-251834 has suggested an electrostatic relay comprised of a frame-shaped base, a movable contact electrode including a movable electrode bridging over the base, and having a movable contact projecting from the movable electrode and movable electrode pieces in the form of a comb projecting from the movable electrode, a fixed electrode having fixed electrode pieces in the form of a comb, in mesh with the movable electrode pieces in non-contact condition, and a fixed contact engageable to a top surface of the movable contact. The movable contact electrode makes horizontal slide movement due to an electrostatic force generated by applying a voltage across the movable electrode pieces and the fixed electrode pieces to thereby cause a top surface of the movable contact to make contact with or separate away from a top surface of the fixed contact.
The above-mentioned problems remain unsolved even in the micro-machine switches or other devices suggested in the above-mentioned Publications.